This application is based on and incorporates herein by reference Japanese Patent Application No. 2001-238979 filed on Aug. 7, 2001.
The present invention relates to an alternating current (AC) power generating apparatus that may be used for driving an electric load such as a discharge lamp or the like.
In a conventional AC power generating apparatus such as a discharge lamp lighting apparatus disclosed in U.S. Pat. No. 6,201,350 B1, a DC-DC converter circuit and an inverter circuit are used. The DC-DC converter circuit includes therein a smoothing capacitor for elimination of noise signals, that is, ripple signals. An aluminum electrolytic capacitor is used as the smoothing capacitor, because the electrolytic capacitor is inexpensive and has large capacitance.
When current including large ripples flows in the electrolytic capacitor, the capacitor heats due to its internal series impedance in proportion to the square of the current including the ripples. As the capacitor heats, the internal pressure of the capacitor rises and tends to cause leak of the electrolytic solution in the capacitor. For this reason, the capacitor must be used under the predetermined allowable ripple current condition. As the allowable ripple current of the capacitor is depends on the volume of the capacitor, the capacitor for the lighting apparatus is necessarily sized large to sufficiently perform ripple eliminating performance.
The lighting apparatus further uses a transformer that forms a closed circuit with the capacitor and a switching transistor. The area surrounded by the parts forming the closed circuit is preferably small to minimize electromagnetic radiation noise. However, the large sized capacitor makes it impossible to reduce the area of the closed circuit. As a result, the electromagnetic radiation noise must be reduced by other measures such as electromagnetic shields.
It is therefore an object of the present invention to provide an AC power generating apparatus that supplies AC electric power to an electric load such as a discharge lamp stably in compact construction.
It is another object of the present invention to provide a discharge lamp lighting apparatus that reduces electromagnetic radiation noise.
According to the present invention, an apparatus comprises a smoothing circuit with an electrolytic capacitor therein for smoothing ripples included in a DC voltage supplied from a power source. The apparatus further comprises a switching device and a transformer for transforming a smoothed voltage by turning on and off application of the smoothed voltage to the transformer thereby to supply AC electric power for activating an electric load such as a discharge lamp. A capacitor is connected in parallel with the electrolytic capacitor.
Preferably, the parallel capacitor has an equivalent internal resistance that is smaller than that of the electrolytic capacitor. The parallel capacitor has a capacitance that is more than 2% of that of the electrolytic capacitor, and is a laminated ceramic type. The parallel capacitor is located inside an area surrounded by the transformer, the switching device and the electrolytic capacitor, so that a circuit area where electromagnetic radiation noise is generated is reduced.